Application timing critical for peanut fungicides

You can count on at least two things if you're growing peanuts in the Southeast — you'll get diseases, and you'll have to treat for these diseases with fungicides.

“You can't get by without treating for peanut diseases in the Southeast,” says Bob Kemerait, University of Georgia Extension plant pathologist. “Our weather patterns in the Southeast are conducive not only for leafspot diseases but also for soil-borne diseases of peanuts.”

Growers continue to see improved varieties with various levels of disease resistance and tolerance, he adds. “But I don't think we'll ever be able to grow peanuts economically in Georgia without using fungicides. Fungicides are a primary aspect of peanut production in the Southeast,” he says.

The timing of fungicide applications, says Kemerait, is critical to their ultimate success. “When is the most effective time to put out a fungicide? You have to decide when to put them out and get your best return on the investment,” he says.

Early and late leafspot, he says, are fungal diseases that are sensitive to environmental conditions. Warmer temperatures, he adds, equal more leafspot disease, and it's always warm enough in south Georgia for leafspot.

Moisture also is a factor, he says. “The drier the conditions, the less leafspot disease you'll have. The wetter the conditions — like this past year — the more leafspot you'll have.”

Soil-borne diseases also are environmentally sensitive, notes Kemerait. “Soil-borne diseases are affected by soil temperature. Soil temperature especially is a critical factor for white mold. Soil-borne diseases also are affected by soil moisture, irrigation or rainfall.

“Whether you're talking about soil-borne diseases like rhizoctonia limb rot or white mold — or leafspot diseases — you need to be aware of rainfall patterns and irrigation, and how these factors affect the timing of your fungicide applications.”

Most growers, says Kemerait, tend to focus on a 14-day calendar schedule for applying fungicides. They begin spraying about 30 to 35 days after planting and then spray every 14 days. That's an effective method, he says, but it's not based on environmental conditions.

One method that is based on environmental conditions is the AUPnut leafspot advisory developed at Auburn University. This advisory uses the number of rain events and the five-day forecast to stretch or shrink the time between fungicide applications. An advisory is generated based on the number of rain events — 24-hour period with more than one-tenth inch of rain and/or irrigation or fog beginning before 8 p.m. It also uses the five-day average rain probability forecast, and the rain forecast for each day within that five-day average.

“If you choose to use an advisory such as AUPnut, you have better timing of your fungicide applications because you're putting out fungicides only when conditions are favorable for the development of diseases. You may not need to spray as much in a dry year, and you may need to spray more in a wet year. Using an advisory such as AUPnut also helps you to become a better manager because you pay more attention to conditions in the field.”

University of Georgia researchers looked at AUPnut versus a calendar spray schedule in 13 trials at 45 sites, from 1991 through 1999. Over that nine-year period, the calendar schedule averaged 7.1 sprays per year while AUPnut averaged 5.5 sprays.

Leafspot control, says Kemerait, was slightly better with the calendar schedule. “But yield is what really matters, and in 43 of 45 tests, there was no significant difference in yields between AUPnut and the calendar schedule, and we saved one and a half sprays with AUPnut. This tells us that AUPnut can work and can be effective in making fungicide sprays.“

The effectiveness of AUPnut begs the question of why more growers don't use the advisory, says the plant pathologist. “First, they don't have the same confidence and trust in AUPnut that they do in the 14-day calendar schedule. The 14-day schedule works for them, and it doesn't make sense to go to something else that they're not as sure of.

“But the more important reason more growers don't use AUPnut is the inconvenience of checking rain gauges. Until Doppler Radar became available, you had to check rain gauges every day if you used AUPnut. And most growers have more important things to do than drive around checking rain gauges.”

Kemerait completed a three-year research project this past year looking at using Doppler Radar in conjunction with AUPnut. “We wanted to look at how we could use AUPnut — which can save sprays and make you better managers — and make it easier for growers. That's where Doppler Radar comes in, and we looked at using it instead of rain gauges.”

Researchers worked with the Agricultural Weather Information Service, Inc., (AWIS) Website out of Auburn, Ala. There were eight field trials in the three-year study — four in 2001, two in 2002 and two this past year.

Farmers can register their fields and use the AUPnut advisory at www.awis.com. You'll need the global positioning satellite coordinates of the specific fields to do this.

Kemerait and other scientists checked to see how accurate Doppler radar can indicate the one-tenth inch trigger. They registered 10 sites across Georgia for AWIS Doppler data. These sites also were monitored for actual, real-time weather conditions by the University of Georgia's Automated Environmental Monitoring Network. The monitoring sites and the Doppler radar agreed more than 90 percent of the time in indicating rainfall of one-tenth inch or more, he says.

Included in the study, he says, were all of the fungicides commonly used by growers in Georgia and the Southeast.

In 2002 and 2003, AUPnut called for an eighth leafspot spray compared to the seven required by the calendar schedule, says Kemerait. “The good news there is that you're getting in there with an extra spray when weather conditions are favorable for disease development. The downside is that if you're going to put out an extra application, you had better see some return.”

In the three years of the study, there was no significant difference in yields between the peanuts sprayed on a calendar schedule and those sprayed with the AUPnut advisory.

“At this point, some might say that AUPnut is a failure because it calls for eight sprays, and that's not being made up in yield, at least not in the past three years. But I don't think we should look at it as a failure. First, we've shown that Doppler Radar can be used effectively to determine rain events. In addition, the AWIS site is easy to use, and growers who use have used it were satisfied.

“AUPnut is a tool that helps you to make decisions on your farm. You don't have to follow the rules exactly. My common sense told me at times that I didn't think I needed to spray. If nothing else, AUPnut and Doppler Radar can help you determine whether or not to spray. It can help you make decisions.”

In three years of study, says Kemerait, AUPnut did not save any fungicide applications. “Nine years prior to that, we saved an average of one and a half sprays per year — but that's not the most important thing about AUPnut. Better management is the most important thing.

“The real power behind AUPnut is helping you decide not only when to spray but when to put out that first fungicide application. That's where the real savings will be.”